The M79 "Bloop Tube" was the primary weapon of the American grenadier during the Vietnam War (1955-1975). The weapon gave excellent service for its role of close-in fire support but, since it remained a stand-alone weapon by design, the operator was required to carry a secondary weapon for self-defense - typically a pistol with limited engagement range and man-stopping power. Nevertheless, its greatest asset was the capability to fire various warhead types through 40x46mm projectiles including smoke, flechette and anti-personnel through direct-line-of-sight or indirect-line-of-sight engagement.
During the conflict there grew interest on the part of the U.S. military to develop a better alternative. Colt's Karl R. Lewis responded in 1964 with what became the "XM148" single-shot system intended for mounting under the barrel of a standard service rifle. The weapon, drawn up and prototyped in less than 50 days, fielded a weight of 3lb with an overall length of 16.5 inches. The cartridge type remained the 40x46mm grenade with a rated muzzle velocity of 247 feet per second and maximum range out to 437 yards. A quadrant sighting device was fitted for ranged accuracy.
U.S. military authorities appreciated the type enough to order 20,000 examples and these became available to select troopers before the end of 1966. It was used operationally by both American and Australian special forces into 1967 but, before the end of the year, inherent deficiencies in its design were readily noted - the XM148 relied on multiple small parts and its trigger arrangement was such that it could be prone to accidental discharge when the trooper was on the march through the jungle. These issues led to the launcher being removed from service - the cost of a hasty prototyping and development period.
The XM148 was officially superseded by the well-received AAI M203 40mm Under-Barrel Grenade Launcher (UBGL) which is still in widespread service today (2016) though stocks of the earlier weapon remained in training roles for a few decades longer.